In a conventional SPECT study of an organ such as the heart, a radioisotope (Tc-99m, Tl-201, for example) is administered to the patient and the radioisotope is taken up by the organ. Then, the patient is placed in a scintillation camera system and one or more scintillation camera detectors are incrementally rotated about the long axis of the patient, acquiring projection data at each of a number of angular positions or “views” with respect to the patient's body. The detectors acquire project data through interaction with gamma photons which emanate from the patient, and the resulting projection data from each view are processed to obtain image reconstruction data, which reconstruction data forms three-dimensional images (e.g., “SPECT images” or “tomographic images”) of the distribution of the radioisotope within the patient, and thereby of the internal organs and/or biological functions of the patient, such as blood flow.
According to current protocol (e.g. DICOM), the length of time that a scintillation detector remains at any given angular position relative to the patient (“dwell time”) is constant throughout a given scan. That is, a user may specify only a single scan speed or dwell time, which determines the time that a detector remains at a specific view angle acquiring “counts” or gamma absorption events in the detector, which are subsequently used in image reconstruction.
However, because of attenuation and finite collimator resolution, the projection image information content can vary significantly from view to view, i.e., from one projection angle to the next. For instance, the attenuation parameters of the patient's tissues may vary depending upon the angle from which they are viewed, which means that gamma photons emanating from one particular view angle have an effective count rate that is different from the effective count rate at another view angle. Further, finite collimator resolution means that increases in clinically significant image resolution may be asymptotic as a function of time. For example, as shown in FIG. 1, for a region of interest (ROI) 101 of a patient 102, for a given dwell time dt, the number of counts N1 acquired at view angle θ is greater than the number of counts N2 obtained at view angle θ+π.
Therefore, the dwell time at each angle may be either longer or shorter than is necessary or appropriate to gather sufficient projection data to generate the PET or SPECT image, depending upon the characteristics of the particular ROI being imaged at the particular view angle. Under the existing protocols, this means that image quality may suffer because only a single scan speed or dwell time is specified. For example, insufficient projection data may be acquired at certain view angles because the given dwell time is too short for the attenuation characteristics associated with that angle, while scan time may be wasted at other view angles because additional projection data acquired after resolution limits have been reached.
In various clinical or research applications, there may be restrictions on the amount of time available to conduct an image study. Accordingly, improvement in the efficient use of available study time may also enable the overall acquisition time to be shortened, thereby allowing an increase in patient throughput for clinical studies.